Heretofore, in some cases, gas accumulates in secondary batteries such as lithium ion secondary batteries due to overcharge, excessive temperature rise, breakage by external force, and other reasons. That gas may increase the internal pressure of the secondary battery. Accordingly, a battery case of the secondary battery is usually formed with an explosion-proof valve. This explosion-proof valve is designed to break in case the internal pressure exceeds a predetermined value, thereby releasing the internally accumulated gas to the outside. There is also an explosion-proof valve designed to not only release gas but also break a current path at the valve position.
Furthermore, some of the secondary batteries are provided with a current breaking structure in addition to the explosion-proof valve for releasing gas (refer to e.g. Patent Literature 1). This Literature proposes a cylindrical secondary battery formed with an explosion-proof valve at one end side and a current breaking structure at the other end side. This current breaking structure is configured to swell the bottom wall of a battery case due to the increased internal gas pressure. Accordingly, when the internal pressure reaches a predetermined pressure, a welded portion at the center of an inner surface of the bottom wall is separated. This cuts off the current before the explosion-proof valve is broken.
Further, there is also a case where a plurality of the above secondary batteries is connected to each other and used as a battery pack. For instance, cylindrical batteries each including positive and negative electrodes formed at both ends respectively are arranged so that respective tops and bottoms are alternately reversed. The positive and negative electrodes of the adjacent batteries are connected to each other with bus bars.